Thorpe was found lost and confused early yesterday morning in the Sydney street where his parents live, with residents having called police after the 31-year-old tried to get into a car which he mistakenly thought belonged to a friend.

"He became disorientated and he tried to get into what he thought was a friend's car, but it wasn't his friend's car at all," Erskine told the Australian Broadcasting Corporation.

"Obviously someone saw it, or the owner of the car saw it, called the police and they came and realised it was Ian Thorpe." Thorpe was then taken to Bankstown Hospital in an ambulance.

"There was no alcohol involved. He hadn't been drinking or anything like that," Erskine said. "The hospital then suggested — or more than suggested, I think — that he should go into rehab for depression and that's what's happened this afternoon."

Erskine says Thorpe had taken painkillers for a shoulder operation he had last week and also had antidepressants in his system. Thorpe has been living in Switzerland but returned home over Christmas, including visiting Melbourne for the Australian Open tennis tournament.

The former swimmer has previously detailed his experiences with "crippling depression" in his 2012 autobiography, as well as his use of alcohol "to rid (his) head of terrible thoughts".